[CreateSiFiveMetadata][firtool] Memory metadata isn't emitted without "--split-verilog" option

CIRCT version: `72f4c3f7`

### Issue

I'm trying to test out generating Rocket Chip RTL with CIRCT (testing out this PR https://github.com/ucb-bar/chipyard/pull/1239), and I noticed some strange behavior while trying to toggle between enabling the `firtool` option `--split-verilog` and disabling it: when that option is present, the `--repl-seq-mem-file=` option functions as expected, and we get an output file with memory metadata. However, when `--split-verilog` is removed, the memory metadata file isn't generated.

I'm invoking `firtool` in these two ways:

```
firtool \
--format=fir \
-O=release \
--emit-metadata \
\
--repl-seq-mem \
--repl-seq-mem-circuit=TestHarness \
--repl-seq-mem-file=./RocketConfig.mems.conf \
--annotation-file=../RocketConfig.sfc.anno.json \
--split-verilog \
-o ./ \
path/to/file.fir
```
vs
```
firtool \
--format=fir \
-O=release \
--emit-metadata \
\
--repl-seq-mem \
--repl-seq-mem-circuit=TestHarness \
--repl-seq-mem-file=./RocketConfig.mems.conf \
--annotation-file=../RocketConfig.sfc.anno.json \
-o ./output.sv \
path/to/file.fir
```

### Expected Behavior

The file specified by `--repl-seq-mem-file` should be generated, regardless of the presence of the `--split-verilog` flag.

### Reproducing

I'm attaching an archive which contains the input annotation and FIR files, as well as a script to run the test. To run the test, invoke:

```
./run-test.sh
```

Make sure `firtool` is on your PATH when you run this. The test will invoke `firtool` twice, once with `--split-verilog`, and once without. The output with `--split-verilog` will end up in `./output-dir-split`, and the output without `--split-verilog` will end up in `./output-dir-no-split`.

You should notice that the file specified in `--repl-seq-mem-file` gets generated in `./output-dir-split`:

```
$ cat output-dir-split/RocketConfig.mems.conf
name cc_dir_ext depth 1024 width 136 ports mrw mask_gran 17
name cc_banks_0_ext depth 16384 width 64 ports rw
name data_arrays_0_ext depth 512 width 256 ports mrw mask_gran 8
name tag_array_ext depth 64 width 88 ports mrw mask_gran 22
name tag_array_0_ext depth 64 width 84 ports mrw mask_gran 21
name data_arrays_0_0_ext depth 512 width 128 ports mrw mask_gran 32
name l2_tlb_ram_0_ext depth 1024 width 44 ports rw
name mem_ext depth 512 width 64 ports mrw mask_gran 8
```

However, that file isn't present in `output-dir-no-split`:

```
$ ls output-dir-no-split
RocketConfig.sv
```

Before re-running the test, make sure to remove `output-dir-no-split` and `output-dir-split`.

[circt-test.tar.gz](https://github.com/llvm/circt/files/9790916/circt-test.tar.gz)

Start with the attached archive and run run-test.sh using firtool on PATH, comparing output-dir-split with output-dir-no-split. Trace the differing behavior for --split-verilog and --repl-seq-mem-file; done means the specified memory metadata file is generated in both output directories.

Written by the indexing model from the issue text.